In general, an ink jet head of an ink jet printer has a driver IC for driving the ink jet head. Print data is usually transferred to the driver IC as serial data. As a method of data transfer, transfer of packet data, including a start bit and data to be transferred subsequent thereto is employed. The start bit is detected by the driver IC, and when the start bit is detected, transfer of the data subsequent to the start bit is performed. Such a start bit signals transfer of both print data and setting data of the driver IC for changing a drive control condition.